An optical sub-module is provided within an optical communication system and used to emit light signals into a transmission medium. An optical sub-module, which emits light signals withholding the information of an electrical data pattern applied at the input interface of the sub-module, may be referred as a transmitter. An optical sub-module, which has a part receiving light signal and converting its information into an electrical signal can be referred as a receiver. An optical sub-module, which has a part being referred as a transmitter and another part being referred as a receiver, may be referred as a transponder.
Transmitters can use several options of modulation formats to imprint the information of the electrical data on the light signal. One modulation format is phase shift keying. With xPSK, the information is imprinted on the phase of each symbol of an optical carrier. The number of discrete phase levels on which the information is mapped can be referred as the order of PSK modulation. One example is binary PSK modulation, which can also be referred as PSK modulation of order 2 or 2PSK. Another example is quadrature phase shift keying (QPSK), which can also be referred as PSK of order 4 or 4PSK. For QPSK, one symbol has four possible phase values (0, π, π/2, 3π/2 in radians, modulo 2π), mapping therefore two bits of binary data on one symbol of the light signal.
The xPSK modulation of an order equal to or higher than four may be referred as high order xPSK modulation. For high order xPSK modulation, each symbol of light signal has more than two states. Therefore, each symbol of light signal withholds information for more than one bit of binary electrical information. Thus, the bit rate of the input binary electrical data of a high order xPSK transmitter and the symbol rate of the light signal emitted by the transmitter will be different. This assertion is true in terms of bit period and symbol period.
A particular form of xPSK modulation is differential phase shift keying of order x (DxPSK), where the data information is not mapped directly on the phase levels of the light signal, but on the difference between the phases of consecutive symbols of the light signal. The difference between an xPSK transmitter and a DxPSK transmitter of the same order is the mapping of the information. However, other aspects of the transmitters, including the modulation of the light signal, can be identical. Therefore, when the data mapping method is not specified, DxPSK transmitters can be referred as xPSK transmitters.
For a high order xPSK transmitter, the information is mapped on more than two phase values. There are three schemes of phase modulation for these transmitters. In the first scheme, one single modulator device imprints all possible phase values. Parallel modulators usually comprise several parallel arms, on which the input optical carrier is coupled, phase modulated independently by each arm, shifted by a fixed amount of phase depending on the arm, and mixed. The resulting modulation phase is a vectorial summation of the phase vectors of each arm. This scheme has the advantage of integration but these devices are difficult to manufacture and voltage driving is not trivial because higher amplitudes are required. In the second scheme, consecutive phase modulators modulate the phase of a continuous optical carrier sequentially. Each modulator can imprint two different phases on the carrier and the light modulated by the modulator travels to the next modulators. Finally, the optical carrier having been modulated sequentially by each phase modulated is emitted into the transmission medium. The resulting modulation phase is a scalar summation of the modulated phases of each modulator. This scheme multiplies the number of modulators but has the advantage of using simple devices with trivial voltage driving. The third scheme is a hybrid scheme of the first scheme and the second scheme. Transmitters, in which the phase modulation is performed according to the first scheme; may be referred as parallel xPSK transmitters. Transmitters, in which the phase modulation is performed according to the second scheme or the third scheme, may be referred as serial xPSK transmitters. For a parallel xPSK modulator, each arm of the modulator must be driven by a voltage corresponding to a set of binary data. For a serial xPSK modulation, a driving voltage corresponding to a set of binary data has to be implemented for each phase modulator. If the coding of binary data is performed outside the transmitter and if the transmitter has an input for each set of binary data required by the modulator driving scheme, no additional coding is required on the transmitter. Otherwise, a coder is implemented on the transmitter, which determines the phase to be modulated by each modulator or each arm of modulator and dispatches an electrical voltage accordingly.
In the case of a serial high order xPSK transmitter, the phase imprinted by each modulator sequentially is desirably synchronized with the light signal travelling to the modulator. Therefore once phase signal has been imprinted to the optical carrier by the first phase modulator, the timing of phase modulation at each following phase modulator is set so that the time difference between the electrical data signal corresponding to the phase to be imprinted by the following modulator and the light travelling to the modulator is null. If the time difference is more than one symbol period in the absolute value, the imprinted phase does no longer correspond to the phase required to map the data and the information of the emitted light signal is corrupted. If the time difference in the absolute value is less than one symbol period but not null, the imprinted phase information is correct but the imprinted phase shifted from the ideal phase has a direct impact on the bit error rate (BER) of the demodulated signal after transmission. In the same way, for a parallel xPSK transmitter, it is desired that timings of the light symbols modulated by the respective arms of a modulator match one another, when they are mixed. Therefore, it is desired that the times during which each binary pattern travels to the corresponding arm and during which each modulated light symbols modulated by the corresponding arm travels to the mixing point match each other. If the time difference is more than one symbol period in the absolute value, the imprinted phase does no longer correspond to the phase required to map the data and the information of the emitted light signal is corrupted. If the time difference in the absolute value is less than one symbol period but not null, the imprinted phase information is correct but the imprinted phase shifted from the ideal phase has a direct impact on the BER of the demodulated signal after transmission.
The ways to change the timing difference are: to change the length of the optical path between consecutive phase modulators for serial modulation, or on each arm of the modulator for parallel modulation, by changing the refraction index or the length of the path; to change the length of the electrical path of, on which the data travel to the consecutive phase modulators for serial modulation or to each arm of the modulator for parallel modulation, for instance using an electrical phase shifter; to use a buffer to delay the binary pattern by a number of binary bits different for each binary data stream; and to use any combination of the precedent ways.
Precise designing and manufacturing of the serial high order transmitter may provide a fair match for the timing; however, it offers no way to guarantee the match within a symbol period for high symbol rates. Moreover, there is no way to optimize the timing. In addition, for higher speed applications, on a constant modulation format, the symbol period decreases, therefore the timing of the phase modulation performed by each phase modulator has to be set more precisely at constant signal quality. Moreover, designing constraints for transmitters may require or may be relaxed by the design of different lengths for electric paths to each phase modulator for serial modulation or to each arm of a serial modulator. Therefore, it is desired that the timing of modulation is carefully calibrated.
When manufacturing, calibrating or setting parameters of a serial high order xPSK transmitter, the timing of phase modulation at each phase modulator following the initial one is desirably set within a symbol period of the transmitter so that the information imprinted on the emitted optical carrier is correct. The optimization of the transmission characteristics of the transmitter requires further setting of the timing.
When manufacturing, calibrating or setting parameters of a parallel high order xPSK transmitter, the timing of phase modulation at each arm is desirably set within a symbol period of the transmitter so that the information imprinted on the emitted optical carrier is correct. The optimization of the transmission characteristics of the transmitter requires further setting of the timing.
When the wavelength of the transmitter is tunable, changing the emitted wavelength changes the optical path of the light signal inside the transmitter. Thus, the timing is desirably set accordingly to the change of wavelength.
Various approaches have been proposed to set the timing of the phase modulation within an xPSK (or DxPSK) transmitter. Japanese Laid Open Patent Application No. JP-P2007-43638A discloses a technique for setting the timing of the phase modulation for a parallel RZ-DQPSK (return to zero QPSK) transmitter. In this transmitter, a low frequency signal with a frequency of f0 is added to the driving voltage of each modulator arm, and the 2f0 frequency component of the output light signal is detected by a photo detector and a band pass filter (or a low pass filter). One of the modulator arms incorporates a phase shifter, and the phase shift of the phase shifter is controlled in response to the 2f0 frequency component of the output light signal.
Japanese Laid Open Patent Application No. JP-P2007-82094A also discloses a technique for setting the timing of the phase modulation for a parallel RZ-DQPSK (return to zero QPSK) transmitter. In this transmitter, a frequency component of the output light signal in a predetermined frequency range other than the symbol frequency and the harmonic frequencies thereof is detected by using a photo-detector and a band pass filter (or a low pass filter). The phase shift of a phase shifter within a modulation arm is controlled in response to the detected frequency component. Japanese Laid Open Patent Application No. JP-P2007-329886 discloses a similar technique in which the timing of the phase modulation of each modulation arm, instead of the phase shift of the phase shifter, is controlled in response to frequency component of the output light signal in a predetermined frequency range other than the symbol frequency and the harmonic frequencies thereof.
Furthermore, Wu et al., in “Experimental Synchronization Monitoring of I/Q Misalignment and Pulse Carving Misalignment in 20-Gbit/s RZ-DQPSK Data Generation”, ECOC 2007, paper 3.5.5, present a method which can be used to set the timing of modulation for a parallel QPSK transmitter. However, this method cannot be used to set the timing of modulation for a serial transmitter. Moreover, this method has a narrow tuning range limited to one symbol on the timing.
In addition, Wu et al., in “Experimental Synchronization Monitoring of I/Q Data and Pulse-Carving Temporal Misalignment for a Serial-Type 80-Gbit/s RZ-DQPSK Transmitter”, OFC 2008 paper OTuG2, present an other method, which can be used to set the timing of modulation for a serial 4PSK transmitter. However, this method has a narrow tuning range limited to one symbol on the timing and requires an optical spectrum analyzer, which is bulky and expensive measurement equipment.
Japanese Laid Open Patent Application No. JP-P2008-48150A discloses a technique for detecting and adjusting the delay and gain mismatch of a delay interferometer within an optical receiver. In this technique, differential output light signals of the delay interferometer are detected by a differential photo-detector pair and the output of the differential photo-detector pair is analyzed by a spectrum analyzer.
However, there is room for improvement in configuration simplicity, tuning range, and speed of the timing setting of the phase modulation within a high order xPSK transmitter. There is a need for simple, fast and wide-range setting of the timing of phase modulation, which can be used for both serial and parallel types of high order xPSK transmitter.